Seymour is absent in "Raise High the Roof Beam, Carpenters" and dead in "Seymour: an Introduction," yet we know a lot about his character through Buddy's careful narration. Salinger very cleverly gets Seymour's voice across to us by including written excerpts – of his diary, his notes to Buddy – and descriptions of Seymour's writing (namely his poetry). Combine all of this with everything we know about Seymour from the other Glass family tales – Franny and Zooey and "A Perfect Day for Bananafish," and we get a solid picture of the man hiding in the shadows of the prose.
Seymour was a child genius who earned his Ph.D. in his teens and then became an English professor at Columbia University. He was a poet who wrote in a form of his own crafting, the double-haiku. Seymour served in the army and suffered what we might today call Posttraumatic Stress Disorder. It is implied that he attempted to commit suicide by cutting his wrists, but survived the suicide attempt some time before his marriage to Muriel. As the oldest in the Glass family, Seymour was teacher, guide, guru, and arbiter to his six younger siblings. An intensely spiritual soul, Seymour combines elements of different religions – including both Christianity and Zen Buddhism – to form his own personal religious philosophy. Due to some combination of his spirituality, intelligence, wisdom, and life experiences, Seymour is socially detached. He has difficulty relating to people outside his own family, including his fiancée Muriel.
The relationship with Muriel is an interesting one. She seems like an odd choice – possibly a very bad choice – for a man like Seymour. While Seymour is spiritual, detached, and thoughtful, Muriel is rather shallow and extremely social. Though we don't get a great picture of her character in "Roof Beam," we know from "A Perfect Day for Bananafish" that she's the opposite of Seymour. We believe that she loves him, and that he loves her (he insists on as much repeatedly in his diary), but it's pretty hard to see why.
That's where Charlotte Mayhew comes in. From the bits and pieces we hear, it's clear that Seymour and Charlotte had an intense childhood love for each other. Their love was adorably manifested in very childish ways (Charlotte used to stomp on Seymour's foot when he said something she liked, much to his delight). When we find out that Muriel looked very much like Charlotte as a child, we're tempted to jump right to an "Aha!" moment. To make sure we notice how important this detail is, Salinger emphasizes Buddy's reaction: "I couldn't quite take in this information whole," he says, "let alone consider its many possible ramifications" ("Roof Beam" 4.40).
The "many ramifications" include the idea that Seymour thinks he is in love with Muriel because he is really in love with Charlotte. Subconsciously, he can pretend that Muriel is Charlotte because of their childhood resemblance. This is definitely a Freudian interpretation of Seymour's relationship. (Freud was an psychiatrist and the founder of psychoanalysis, and he gets more than a few mentions in Glass family stories.)
But, just how Salinger – and in turn, Buddy – feels about Freud isn't exactly clear. On the one hand, Buddy resents that Seymour was subjected to the poking and prodding of "every summa-cum-laude Thinker and intellectual men's room attendant" who felt like psychoanalyzing him, Freudians who "stopped just short of taking a brain smear from him" and even then only because they ran out of time ("Roof Beam" 3.15, 2.1.2). Freud is portrayed as pop psychology, embraced by pseudo-intellectuals like Mrs. Fedder. And in Franny and Zooey, Zooey pokes fun at Freud by pretending to psychoanalyze Franny's dream while holding a cigar in hand.
On the other hand, Buddy gives Freud the highest praise in "Seymour" – he calls him a poet: "Surely the one and only great poet the psychoanalysts have had was Freud himself; he had a little car trouble of his own, no doubt, but who in his right mind could deny that an epic poet was at work?" ("Seymour" 1.2).
This leads us to two possible interpretations of the Muriel-Charlotte connection. Either Salinger does in fact respect Freud and does intend this Freudian explanation for Seymour's love. Or Salinger dislikes Freud, in which case this explanation is meant to be ironic.
Let's move on, then, and look at Seymour's reasons for missing his own wedding. It's quite simple, he says – he's too happy to get married. As the Matron of Honor so angrily points out, this doesn't make any sense. How can anyone be too happy to get married?
This brings us back to the idea of Zen detachment, as embodied by the elder uncle, the deaf-mute, which we discuss in "Symbolism, Imagery, Allegory." Seymour can't maintain the ideal Zen detachment if he's caught up with floods of emotion (marital bliss, in this case). It compromises his spiritual well being. To understand this concept better, you might want to try reading "Teddy," in which a very precocious small boy expounds on the benefits of emotional neutrality.
One more point before we go. We have to remember that everything we know about Seymour comes to us through Buddy. By claiming ownership of the other, third-person-narrated Glass stories, Buddy warns us that we see Seymour through Buddy's very subjective lens. In some cases, we might even think we're seeing Seymour when what we're really seeing is some aspect of Buddy's own character. This is the case with "Bananafish," he confesses – the character of "Seymour" is actually more like Buddy than he is like Seymour.
You could correctly point out that we get glimpses into Seymour's own writing, his diary, for instance, which Buddy swears he typed out word for word from the original. Again, you have to remember that Buddy is choosing which excerpts to show us, as well as situating them and commentating on them in a way that disallows for any objective, un-Buddy-tainted viewing on our part.
As readers, we can be bothered by this problem of subjectivity, or we can see is at a necessary part of our understanding of both Seymour and Buddy. Remember Seymour's claim, in his letter to his brother, that the two of them were actually quite similar – that the boundary between them was blurry at best. We can't get at Seymour's character without also getting at Buddy's, and vice versa. Who better to attempt this grand undertaking – a portrayal of Seymour – than his brother Buddy? The difficulties of narration and subjectivity are not barriers to these works, but are integral part of these works. That's what makes "Seymour: an Introduction" so important to the Glass family saga – it's where these issues are explicitly addressed for the reader.